More Shell Scripting

Last time: Shell Programming Lab

Sometimes you want to be able to pass multiline data into a program.
If that data is in a file, you redirect stdin (e.g., with <)
If that data is the output of another program, you use a pipe to connect them
But what if you want to dynamically generate that data?
- You could use something like "echo line-n >> file"
- and then run as "program < file"
- A better answer is the "here document"

The here document provides the ability to send inline text to a program until a user-defined terminator is reached.
To use the here document, after the command receiving the input we place a << symbol and the name of the terminating string. The text to be used follows, terminated by the same user-defined string on a line by itself.
```
$ cat << FINISH
> Hello $LOGNAME
> Time is $(date +%T).
> FINISH
Hello brian
Time is 13:21:33.
```

Positional parameters can be reset using the set command

#!/bin/bash
echo Script was called as $0 $*
oldargs=$*
set new1 new2 new3
echo New params are $0 $*
set $oldargs
echo Restored $0 $*

The example of a function we saw previously was not much more than a macro.
In fact, bash functions can take parameters, use local variables, and return (integer) values.

Arguments are interpreted inside the function as positional parameters.

#!/bin/bash

function Usage { echo "error: $*" 2>&1; exit 1; }
if (( $# != 2 ))
then
   Usage "$0: requires two arguments"
fi

The local keyword declares a local variable
return allows you to return a single value between 0 and 255 (just like an exit code)

After executing a function or a program, the exit status is stored in $?

#!/bin/bash

increment() {
   local sum;
   let "sum=$1 + 1"
   return $sum
}

increment 5
echo $?

It turns out that you run shell scripts all the time without knowing it.
When you login, the system runs multiple scripts, including the one in /etc/profile
The startx program (which starts X-windows if not already running) is a script.
Many others, such as netscape, ps2epsi, and a number in /usr/bin/ are scripts, too.
Let's look at a few of them.

I "grew up" with tcsh
Both tcsh and bash are modern, powerful shells.
Both support
- Aliases
- Command-line editing
- Directory stacks
- Filename completion
- History
- Job control
- Key binding
- Prompt formatting
In addition, bash supports functions!
For some details of the differences in syntax, see Quigley Appendix B and this quick online chart